Thiswas a brief exploratory visit to determine the feasibility of using the ion-selective hydrogen probe to localize proton pumping from mitochondrial rich (MR) cells in the frog skin. Frog skin physiology has long been a model system for the study of transport mechanisms. These mechanisms are most frequently studied using conventional Ussing chamber configurations, which permit the detailed study of populations of cells but not the examination of individual cells amongst a population. Dr. Jensen's visit was precipitated by his research at the Renal Unit of Mass. General Hospital. where he learnt of our vas deferens studies with Breton and Brown. The problems addressed in that study are similar to those in the frog skin, particularly in regard to the role of the V-type proton pump. Application of the probe technology offers a number of clear advantages, notably the sensitivity, ion-selectivity and the ability to position the probes directly over the morphologically distinct MR- cells. Using the upright microscope configuration (funds received in the last renewal) we are ableto place the electrode, oscillating in the Z-plane, in close apposition to the MR cells. A localized proton derived signal is recorded although work will still be necessary to refine the preparation. The success of this brief visit encouraged Prof. Larsen (Jensen's supervisor) to install a Seris system in the August Krogh Institute in Denmark.